This invention relates generally to magnetic resonance imaging magnets and more particularly concerns a non-enclosed magnet arrangement which combines refrigerated superconducting coils with ferromagnetic pole pieces.
Magnetic resonance imaging (MRI) is now a widely accepted medical diagnostic procedure and its use is becoming increasingly popular. MRI systems require a uniform magnetic field and radio frequency radiation to create an MR image. Various types of magnets are currently used to produce the magnetic field. Whatever type of magnet is used, it is necessary that a portion of the magnetic field be highly homogeneous. This portion of the magnetic field, referred to herein as the imaging volume, is the portion of the field which covers the subject being imaged. Field homogeneity in the imaging volume is necessary to obtain a quality image. Therefore, it is advantageous for many imaging applications to have a magnet which can produce a relatively large imaging volume.
Typically, magnets used for whole body imaging are arranged so as to be contained within a cylindrical structure having a relatively narrow central bore. The narrow bore forms an enclosed chamber into which a patient must enter for imaging. These cylindrical arrangements provide many positive features, but there are some circumstances in which they present difficulties. For example, a significant percentage of patients are sensitive to the closed nature of conventional MRI systems. For them, the MRI process can be uncomfortable or even unbearable. Furthermore, some patients are simply too large to fit into the narrow chamber of a conventional MRI system. Difficulties can also arise in placing a patient in the enclosed chamber if the patient is required to remain connected to an IV system or other medical equipment. The closed conventional systems are also difficult for veterinary applications because many animals are frightened by the closed chamber.
Various open magnet arrangements have been proposed which present more accessible alternatives to the closed, conventional systems. One such magnet arrangement is described in U.S. Pat. No. 4,875,485, issued Oct. 24, 1989 to Kinya Matsutani which comprises two superconductive coils arranged in a spaced-apart, parallel relationship to define a working space therebetween. The relatively open space is well-suited to receiving patients and is not likely to induce claustrophobic reactions. This arrangement is said to create an spherical imaging volume having a diameter of 20 cm with a homogeneity of 50 to 100 ppm.
U.S. Pat. No. 4,924,198, issued May 8, 1990 to Evangelos T. Laskaris shows another open magnet arrangement in which two magnet assemblies are arranged in a spaced-apart, parallel relationship. In one embodiment, each magnet assembly comprises a superconductive coil and an inboard resistive coil situated substantially coplanar and concentrically therewith. This embodiment with a 0.5 Tesla central field presents a spherical imaging volume of 20 cm with a peak-to-peak homogeneity of 30 ppm. In a second embodiment, each magnet assembly comprises a pair of superconducting coils. This embodiment presents a spherical imaging volume of 25 cm with a homogeneity of 13 ppm at 0.5 Tesla.
U.S. Pat. No. 5,153,546, issued Oct. 6, 1992 to Evangelos T. Laskaris also shows an open magnet arrangement having two parallel, spaced apart superconductive coils. This device also includes a pivot mechanism which permits the pair of coils to be rotated between horizontal and vertical positions. With a field strength of about 0.3 to 0.5 Tesla, this arrangement provides a spherical imaging volume of 30 cm with a homogeneity of about 30 ppm.
The above open magnet arrangements produce imaging volumes ranging from 20-30 cm in diameter. While these arrangements are effective, it would be advantageous to have an open MRI device which could produce an even larger diameter imaging volume while still maintaining a sufficient level of homogeneity.